Sizing a solar system - newby questions

All,

I'd like to add solar power to our boat but am unsure of how many watts. My goal is just to significantly cut down generator run time. We have a huge almost flat roof that we can't do anything with because of a low fixed bridge, which makes it perfect to add a bunch of solar panels. So, space isn't a concern. Cost is a concern but if we're going to do it, we're going to do it right, not skimp to save a few bucks. We just moved aboard anchored out and are still learning about living aboard, what we use and don't use, what's important to us. We live in Florida and may go further south. We have a great 16kw Kubota generator, which we have no problem running, but we both enjoy the quiet of no generator. We have 12 6 year old 6v golf cart batteries wired in series and parallel for 2 12v banks. We do not yet have a working inverter.

We are normally running 2 80w Edgestar chest fridges/freezers, one dual monitor desktop computer (~500w?), a laptop (200w?), a stereo (no idea what it really draws because it sure doesn't put out what it was advertised to), two wireless routers, the VHF and chartplotter, and a few LED lights. We have a window A/C unit, a rooftop A/C unit, electric hot water heater, and an electric stove but do not plan to run these off an inverter. A quick calculation tells me we normally draw ~700-1500w depending on the desktop computer.

I'm thinking about a 1000-1200w solar system and expecting ~70% efficiency (just guesstimates). I figure we will still probably run the generator once a day or every other day to fire up the stove, top off batteries, or for hot water if needed. Maybe more when it gets warmer for a/c, but we're in the panhandle on the coast so we normally have an awesome breeze.

Am I in the right ballpark for solar system size? I realize that since we have the generator the system doesn't need to be perfectly matched, but I'd hate to discover we have to turn on 12v appliances to burn solar power or we're still running the generator a bunch.

What happens when you've got a relatively large solar setup but you also have access to shore power? Unplug from shore power and use the solar system to avoid too much power? (I'm not sure how to phrase this but hopefully am making myself clear).

Any thoughts on a system such as this:
1200 Watt 12 Volt Polycrystalline Cabin Kit | Renogy Solar

Please do not hesitate to point out what I have missed or suggest alternatives. We are still learning and I'm open to constructive criticism! Thanks in advance for the help.

Panhandler,
I'm new at solar as well and will be installing this summer. Kyocera KU265'S are available at $250 each, I will be installing 6 of them for 1,590 watts. They will be run thru 2 mppt Midnite or Outback charge controllers.
The altestore has a whole series of video's and the Midnite solar site has a good size calculator. I look forward to responses to the thread and keep us up to date as you go along,
Arch

I think the trick on a boat is to get as much power as you can from the available space. In that respect, it sounds like you have a distinct advantage that will only make the rest of us envious. I'd suggest picking panels that will give the max coverage (slightly different shapes can make a big difference), and use an MPPT charge controller. Also, watch out for even minor shading. Small shadows can make a huge dent in power output. It's also beneficial to wire as many panels in series as you can without exceeding the voltage rating of the controller. That will reduce wiring sizes and generally creates greater immunity to shading.

twistedtree said:

I think the trick on a boat is to get as much power as you can from the available space. In that respect, it sounds like you have a distinct advantage that will only make the rest of us envious. I'd suggest picking panels that will give the max coverage (slightly different shapes can make a big difference), and use an MPPT charge controller. Also, watch out for even minor shading. Small shadows can make a huge dent in power output. It's also beneficial to wire as many panels in series as you can without exceeding the voltage rating of the controller. That will reduce wiring sizes and generally creates greater immunity to shading.

Click to expand...

+1. I envy the space.

Done a couple of rigs over the last decade or so. There are many factors that require attention and the most important is storage, Without decent storage capacity energy production is does not mean a lot. Work out your requirement, add 20%. Check the boat for storage areas that are cool, well ventilated. Keep cable sizes high, cable runs short. Wire panels to provide high DC voltage. Mavericks will handle up to 600V DC if I remember. As a rough Guide 1kw of panels works well with 1000amps of storage (Say 10 T105+'s).Check your local WX station sunshine history / or where you'll be cruising etc. Have a realistic feel for what you can produce. make sure you go with a reputable MPPT controller. Maverick are excellent, easy to use. Most of all make sure the batteries are connected correctly with the Pos and Neg as far apart as possible in the bank (opposite ends) otherwise the bateries closeest to the final anodes will discharge deeper than the furthest from inverter. Fortunately lots have advancements have been made over the last decade, incredibly better controllers, more efficient panels that will even give out voltage in moonlight and streetlights etc.
The only reason I start the genny on my boat is to use 220vac. (Galley top / Aircon)

My bank is on Float or absorb by afternoon, and charging starts 30 mins after dawn. I don't need sun (40-60amp Great charging) just a bright day (20-30amp Fair charging)

Hopefully this brief bit of info is of use to you. If you wish to chat about your project P.M. me and I'll give you a Skype/ring. I'm currently cruising in the eastern caribbean

.

There is a device that measures energy usage in watts, amps, and kilowatt hours called a Kill A Watt. Link: http://www.amazon.com/P3-International-P4460-Electricity-Monitor/dp/B000RGF29Q You can use this to measure the actual consumption of any 115v device. This helps greatly to estimate energy usage. I found that my MacBookPro only uses 18watts when running on 115v.

Larry H said:

There is a device that measures energy usage in watts, amps, and kilowatt hours called a Kill A Watt. Link: http://www.amazon.com/P3-International-P4460-Electricity-Monitor/dp/B000RGF29Q You can use this to measure the actual consumption of any 115v device. This helps greatly to estimate energy usage. I found that my MacBookPro only uses 18watts when running on 115v.

Click to expand...

A cheap bastard like myself realized I only needed it once and found that my local library checks out "Kill A Watt" meters.

To really know what you need, you might consider finishing planning the electrical load side before computing what you'll need on the charge side of the equation. Do you have a particular inverter size or model in mind?

If you get an AC load meter like the Kill-A-Watt, you can find the load by switching the stereo on and off while watching the meter.

"A cheap bastard like myself realized I only needed it once and found that my local library checks out "Kill A Watt" meters."

Craig, you could have borrowed mine. I would have passed it over the rail, saving you gas money to the library. You blew it!

I hope Dswizzler sees this and checks in. I just saw a FB post of his today that showed he was getting 81A for 6-8 hrs per day. The meter indicated 81.6A, 69.9V, 3.7 kWh and 1077W! His batt was 13.3V using an MPPT controller. He's in a southern latitude in Baja California, Mexico. Very impressive figures.

Thanks, all. I appreciate the help! Seems like it's time to buy a Kill A Watt.

I would approach this differently than others have suggested with a Kill-a-Watt meter.


I would install an inverter probably 2,000 watts and a battery monitor. Then with your batteries fully charged (and paralleled- you don't need separate banks) run everything from the inverter for a typical day. The energy monitor will tell you how many amp hours of DC was required to power all of it- AC from the inverter and DC for lighting, etc.


Then base your decision on that value. Let's assume it is 200 amp hours. As a rule of thumb a typical flat solar panel will put out 1/3 of its watt hour rating on a sunny day in amp hours at 12V. So that means 600 watts of panels. But you need about 25% more due to Peukerts equation (losses in battery charging), so make it 750 watts. To be really comfortable and to deal with cloudy days, you might want to install 1,000 watts.


You probably will want to install 200+ watt panels, so that means the higher voltage ones that produce their maximum power at about 35 volts and have a Vmax rating of about 45. Pick a MPPT controller with a Vmax rating of 45 volts or more and an output current rating of the Imp of the panel times the number of panels plus a 20% safety factor. You might want to buy an even bigger controller if you think you may want to fill up your roof someday.


Then you have to size your conductors correctly, both the panel to controller run and the controller to battery run. Mount your controller near your battery bank to minimize losses and size that conductor to limit the voltage drop to a few tenths. The run from the panels to the controller will be ok with a half volt or so loss.


The design process isn't complicated but it all starts with your DC requirements. Figure that out and the rest is straightforward.


David

I have found very few boats have the install space to take care of the biggest normal daily load the reefer.

For those that do , great.

For most the solar has the best use at topping the batt set.

That last 10% or 15% that would take "forever" to do with mechanical electric.

This is time sensitive , not power sensitive , so about 100W with a smart charge controller is all that's required.

David, that's great advice. I'll be adding an inverter anyway. Thanks!

FF, by my rough calculations we've got space for about 10kw of panels. 1,000 watts is nothing just because of the large almost flat roof.

"FF, by my rough calculations we've got space for about 10kw of panels. 1,000 watts is nothing just because of the large almost flat roof."

At a buck a watt plus he charge controller , an inverter and larger batts it is possible to run a quality reefer.

Check out the Sun Frost , and weather you can get it aboard.

www.[B]sunfrost[/B].com/Sun Frost

IMO, once you put on solar panels, the requirements for sizing of the battery bank changes. A smaller sized bank makes some sense if your goal is to cover your loads during the day, and top up your battery bank. It means less genny time and the bank comes up to 100% SOC faster. The tradeoff is you cannot store more power than the available capacity of your batteries.

I first looked at the ratings of the various loads and projected how much they would be on to get an initial estimate of total daily amp hours used. This was confirmed within 10-15% based on the SOC meter. We found that we used 200-250 amp hours/day.

Our bank is 1125 amp hours. At a daily use of 200-250 amp hours/day while at anchor, we could go 2 days and still be above 50% SOC. With solar, we may not need all of that capacity. We have 435 watts of panels which is all we could accommodate with our available space. I'm getting about 20 amps out of them at noon. Approximately 50% of that goes to running the average instantaneous loads and 50% goes into charging. As the sun's angle changes, the available energy for charging declines until the panels are unable to cover the instantaneous loads. My goal was to reduce genny runtime by 75% and to get the batteries to 100% SOC occasionally at anchor. Long term I hope to increase the life of the battery bank and make some savings on fuel and maintenance of the genny.

Jim
Sent from my iPad using Trawler Forum

Panhandler said:

David, that's great advice. I'll be adding an inverter anyway. Thanks!

FF, by my rough calculations we've got space for about 10kw of panels. 1,000 watts is nothing just because of the large almost flat roof.

Click to expand...

Wow! That's 40 X 250 watt panels at 1.6 metres per panel or about 65 total square meters (700 square feet).


Jim
Sent from my iPad using Trawler Forum

a couple of points from post #1:

1. Liveaboard, anchored out. To me this says go for as much solar as possible.
2. What happens to panel output when batteries get full? Nothing - you do not have to burn excess power!

I'm about to increase to 2070 W in panels and have a 1284AH AGM 12V house bank. Once you have solar you will likely add to it if you have space for panels, so plan for a modular system to make adding more modules at a later date an easy thing to do.

Follow Twisted's advice in post #3 and David's advice in post #10. But go for at least a 3000W inverter.

Once you know daily Ah consumption you might want to use the kill-a-watt unit to identify power hogs. I did, and replaced the pilothouse reefer (just used for drinks) as a result. And I only run the desktop PC (another power hog) for navigation when the engines are running. Use a laptop PC for internet and email, as Larry highlighted, they use very little power.

Thanks, all. More great advice!

JDCAVE, I was a little off. It's more like 615 square feet of available roof space. 52' with a 16' beam and a flat roof which covers most of it!

52' with a 16' beam and a flat roof which covers most of it!

With modern gear you might be able to air condition , say the galley, with that much juice!

Trucks have 24V DC units of about 5000BTU

Wow that's a lot of roof! I'd cover every inch of that with panels. Just noticed the Alte store online has them down to 94 cents per watt. We are starting to build a small off grid place that will be entirely dc and as FF alluded to we are looking at bus/truck 24 volt equipment.

Solar really has come of age and is totally viable if the politicos would just quit trying to help it.

"we are looking at bus/truck 24 volt equipment."

Mostly the air cond makes sense in 24V for the house goodies , as soo much is made in 12V.

Backwoods Solar has a free catalog you should get , as well as a visit to a Big RV store. .

You might consider subscribing to Home Power for lots of off grid info.

Solar heating , hot water , well pumps , legal batt and inverter wiring .,,

Catalog Request | Off-Grid Power | Solar Panels | Backwoods Solar

www.[B]backwoodssolar[/B].com/catalog-request


Everything in our renewable off grid power systems catalog is available on this website but we also offer a printed version and a downloadable PDF of our print catalog for your convenience. ... To request a printed off grid solar systems catalog just email, call, fax or mail your ...

Thanks for the suggestions! While we may have space, I think budget will limit us!

I wonder how the Tesla Powerwall solar battery would work for solar storage on a boat..?

djmarchand said:

I would approach this differently than others have suggested with a Kill-a-Watt meter.


I would install an inverter probably 2,000 watts and a battery monitor. Then with your batteries fully charged (and paralleled- you don't need separate banks) run everything from the inverter for a typical day. The energy monitor will tell you how many amp hours of DC was required to power all of it- AC from the inverter and DC for lighting, etc.


Then base your decision on that value. Let's assume it is 200 amp hours. As a rule of thumb a typical flat solar panel will put out 1/3 of its watt hour rating on a sunny day in amp hours at 12V. So that means 600 watts of panels. But you need about 25% more due to Peukerts equation (losses in battery charging), so make it 750 watts. To be really comfortable and to deal with cloudy days, you might want to install 1,000 watts.


You probably will want to install 200+ watt panels, so that means the higher voltage ones that produce their maximum power at about 35 volts and have a Vmax rating of about 45. Pick a MPPT controller with a Vmax rating of 45 volts or more and an output current rating of the Imp of the panel times the number of panels plus a 20% safety factor. You might want to buy an even bigger controller if you think you may want to fill up your roof someday.


Then you have to size your conductors correctly, both the panel to controller run and the controller to battery run. Mount your controller near your battery bank to minimize losses and size that conductor to limit the voltage drop to a few tenths. The run from the panels to the controller will be ok with a half volt or so loss.


The design process isn't complicated but it all starts with your DC requirements. Figure that out and the rest is straightforward.


David

Click to expand...

I totally agree with this for a land based system where you can generally install as many panels as you need. In that case the questions becomes "how many do you need". But I would be really surprised if the answer was anything other than "install as many panels as you can fit" on your boat. The loads always seem to exceed the available panel space. But let us know if that's not the case. And regardless, understanding your loads is critical either way. It's almost always cheaper and easier to lower your loads than to add more power, and even more so on a boat where you often can't add more power because of those pesky space constraints.

Peter B said:

I wonder how the Tesla Powerwall solar battery would work for solar storage on a boat..?

Click to expand...

They are tempting, but would pretty much require a complete rework of the boat's charging, inverting, and DC power system. The Tesla batteries are high voltage (100-200V if I recall correctly). None of the usual marine alternators, chargers, charge regulators, inverters, battery monitors, or anything work at those voltages. There are industrial devices available, but if you think marine stuff is complex and expensive......

Everyone has had their input into this subject, which is great because I am re-designing my solar setup too. There's been good input on size of banks /batteries vs. amount panels /wattage required vs. daily consumption /load and controllers.
However what about the type of solar panel to harvest all this free energy?
Can I generalize and say there is three types around ATM that are economical, being :-

Monocrystalline
– Good power-to-size ratio: efficiency typically within the range of 135-170 Watts per m2 (13-17%, with notable exceptions).
– Outstanding performance in cooler conditions.
– Some leading units now have over 18% conversion efficiency.
– Previously the most commonly used technology in the world, with over 50 years of technological development.
– Excellent life span / longevity. Usually come with a 25yr warranty.

Polycrystalline
– Good efficiency: typically 120-150 Watts per m2 (12-15%, with notable exceptions).
– Generally speaking, marginally less expensive to produce than monocrystalline.
– Slightly better performance in hotter conditions (lower heat derating coefficient)
– Excellent life span / longevity. Usually come with 25yr warranty.
NB: Monocrystalline solar panels are not necessarily ‘better ‘ or more efficienct than polycrystalline, as many in Australia believe. Read more: Monocrystalline vs polycrystalline silicon solar cells – Busting some myths.

Amorphous Thin Film
– Low conversion efficiency: typically 60-80 Watts/m2 (6-8%, with notable exceptions).
– Expected lifespan is less than crystalline panels.
– Optimal efficiency in hot weather, less effective in cooler conditions.
– 3-6 month ‘breaking in’ period where long term output is exceeded.
– Requires 2-3 times more panels and surface area for same output as crystalline.
– Ideal for example for inland Australia, where conditions are hot and vacant space abounds. (More about thin-film solar cell technology.)
How important is panel efficiency in your system?

Remember that, especially if you have a large roof, panel efficiency may not be the most important thing for you to worry about. It is more important to consider your system as a whole. The ultimate cost and performance of your system will depend not only on the panels you use.

So we have another can of worms to consider..........
My system on 'Nimiane' is a collection of panels both mono and crystalline collected over the years, setup with several PWM controllers to avoid conflict between the different panels - a real dog's breakfast.

What's your take on the type of panel to suit your particular weather conditions or will you just seek a happy compromise. Remembering some of us live in the tropics -ie. Florida/Queensland or the other hand Great lakes/Southern Australia. ?
Looking forward to some /any opinions.

For many boaters a panel that can be partially blocked , and still create juice would be the best choice.

Installed on board efficiency is worth more than a theory.

From what I've seen on boats, there is never enough space to install as many panels as your power needs would suggest. So the trick is to get as much power as possible out of the space available. This translates into a few things.

1) Use monocrystalline panels since they generate the most power per sq meter.

2) Use an MPPT controller to harvest the most power from the panels that you install.

3) Carefully pick panels so that you get max coverage of the available space. Panels come in lots of different physical sizes, and they don't always do a good job completely filling the available space. Picking different panels can get get you 10-20% more coverage and more power. This can be very tedious going through spec sheets and looking at dimensions.

4) Wire panels in series as much as possible. This reduces wiring sizes and number of conductors which is really convenient when you have to pull them through the boat. And it also helps mitigate the effects of shading. This last point can be controversial, and is not always true. Shade tolerance is very specific to the exact panels you select (cell arrangement, blocking diodes, bypass diodes, etc), their physical arrangement, how they are wired, and how shadows are cast over the panels. But for most of us who can only roll the dice and pick series or parallel wiring, I think series is the better bet. You might not win every time in every situation, but I think you will come out ahead over all.

Thanks, I hadn't thought about #4.

Bojgranjac1, good stuff!

IF the cost , expense and install for heavy DC wires ,is a bother the newest panels each have a built in inverter and much higher voltage output , so smaller wiring.